[vm] Fix building with older sysroots.

Cf. dfce3aa0ff

TEST=https://ci.chromium.org/p/fuchsia/g/dart/console
Change-Id: I8c385931548bace9d0753f7dcbae829eff0079bd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/309520
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>

runtime/vm/os_linux.cc

@@ -44,6 +44,11 @@ namespace dart {
 #define ElfW(Type) Elf32_##Type
 #endif
 
+// Missing from older versions of <elf.h>.
+#if !defined(EM_RISCV)
+#define EM_RISCV 243
+#endif
+
 #ifndef PRODUCT
 
 DEFINE_FLAG(bool,